Bubbler

ABSTRACT

A bubble column for facilitating algae growth comprised a tank adapted to contain a volume of growth medium with algae growing therein, with a light pipe disposed centrally therein and extending from near the top surface of the growth medium in the tank to near the bottom of the tank. The light pipe illuminates substantially the entire volume of the growth medium. A diffuser is positioned at or near the bottom of the tank and receives a gas stream, and converts the gas stream to a stream of bubbles that agitate the algae without creating a fluid flow within the tank.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/280,847, filed Nov. 10, 2009, entitled Methodfor Algal Treatment of Effluent from an Anaerobic Digester and Creationof Useful Algal Biomass, which is incorporated herein for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to wastewater treatment systemsand methods, and more particularly relates to wastewater treatmentsystems utilizing anaerobic and aerobic microorganisms forbioremediation.

BACKGROUND OF THE INVENTION

The vast majority of the world's wastewater does not undergo treatmentof any kind before being dumped into the nearest open water source. Thishas resulted in an international health crisis, where people die dailyfor lack of clean water. Unstable ecosystems caused by nutrient richwaste runoff are creating high rates of fish kill, ocean floor plantkill and large concentrations of pathogenic bacteria. This is a directeffect of lack of treatment or poor treatment and disposal of such wastestreams. Effects such as disastrous algae blooms in open water sourcesfrom eutrophic conditions have drastically increased in the past decadeand pose unprecedented environmental problems.

Typical prior art wastewater treatment systems typically employmechanical aeration and chemical treatment. These systems are expensiveto build and to operate, not solely because of the high energy costsincurred in the aeration process, but also because of the manpowerrequired to operate the expensive machinery employed in such systems.Such mechanical/chemical treatment facilities, even those that areconsidered “state of the art,” have a price tag in the millions and evenup to hundreds of millions of dollars, making them so expensive thatmany communities, in the US and other parts of the developed world, havein the past been unable to afford such sewage treatment systems. As aresult, the majority of the world's population lives with massive sewagepollution.

Bioremediation of wastewater has been proposed in the past. Suchbioremediation systems typically employ a combination of aerobic andanaerobic processes. In particular, such prior art systems havegenerally proposed the use of anaerobic bacteria for digestion oforganic matter and the release of biogas, combined with phototrophicorganisms that produce oxygen to accelerate the breakdown of organicmatter by aerobic bacteria. (At the same time, the aerobic bacteriaproduce carbon dioxide which is needed by the phototrophic organisms.)Anaerobic digestion kills most of the pathogenic bacteria found in rawsewage by depriving it of oxygen. In addition, the anaerobic bacteriaare able to digest most of the biologically activating solids. Throughthis anaerobic digestion process, levels of Biological Oxygen Demand(BOD) and Chemical Oxygen Demand (COD) are greatly reduced, in additionto decreasing the amount of solid content in the waste. In this manner,the complementary nature of aerobic and anaerobic processes can beharnessed to break down organic material into its elemental formswithout the use of ‘heat, beat and treat’ systems currently used inconventional, mechanical aeration/chemical treatment waste remediationfacilities.

Algae has long been proposed as a suitable phototrophic organism for usein such bioremediation of wastes. One large project using such anapproach is the St. Helena Wastewater Treatment plant in California, andother such plants have been put into service elsewhere in the world.

These solutions have demonstrated a number of desirable characteristics,but have had significant shortcomings. Because these prior art systemsdo not have a mechanism for controlling the algal specie(s) present,their algae cultures drift over time, often with unwanted outcomes.These undesirable outcomes include the growth of species that cannot beeasily separated from the water at the end of processing; theproliferation of species that grow well during “normal” conditions, butare unable to grow in the case of process excursions, e.g. an influx ofan industrial pollutant; or the proliferation of algaie species thatgrow well, but do not perform all of the desired remediation.

Further, absent a mechanism for active replenishment of the algae,wash-out events (e.g. from a rainstorm) can severely dilute the algaeculture density, such that the system is unacceptably slow to return toan effective culture density.

Thus, there has been a long-felt, and growing, need for a wastewaterremediation system and method that is cost effective while offering anefficient, stable remediation approach.

SUMMARY OF THE INVENTION

The present invention provides a system and method for efficient,cost-effective bioremediation of wastewater and other contaminated fluidstreams. In one aspect, the invention includes a photobioreactor(hereinafter sometimes “PBR” for simplicity) for growing highconcentrations of algae. The PBR comprises a tank having speciallyconfigured light pipes distributed therein to cause high density algaegrowth substantially throughout the tank. Fluid flow in the tank ismaintained at a level low enough to prevent damage to the algae while atthe same time allowing the fluid to circulate throughout the tank.

Another aspect of the invention comprises a medium system for supplyingnutrients to the PBR or other growth system. The nutrient system cancomprise a plurality of separately selected components which are thenassembled into a nutrient stream through a plurality of metering pumps,or, in some embodiments, can be derived from a portion of the effluentof an anaerobic digester. In some embodiments, the anaerobic digesterforms a first stage of the overall bioremediation system. The anaerobicdigester stage, aside from providing a stream rich in micro andmacronutrients, also provides significant amounts of CO₂ to the PBR,which assists in the growth of algae in the PBR. In addition, theanaerobic digester generates significant quantities of biogas, which canbe utilized by a conventional biogas-powered generator to produce atleast a portion of the electricity required to operate thebioremediation system of the present invention. Carbon dioxide from thebiogas can be used in the lagoon or pond to accelerate algae growthbefore, after, or instead of burning of the biogas. In the case wherethe biogas is burned, the resultant heat energy can be used to warm thewater in the lagoon, accelerating various of the desirable biologicalprocesses ongoing there.

Yet another aspect of the invention comprises a remediation lagoon orpond, typically although not necessarily using a raceway design, wherethe remediation pond is fed high-density algal inoculum from thephotobioreactor system. A portion, in many cases the majority, of theeffluent from the anaerobic digester stage provides the incoming fluidstream to be remediated in the remediation pond. In some embodiments,the remediation pond can be a multi-phasic pond utilizing multiplebiological capabilities enabling it to process the residual CO₂,nitrogen and phosphorus remaining in the effluent from the anaerobicdigester stage. In at least some embodiments, the multi-phasic pondscomprise a plurality of horizontal strata, for example: aerobic at thesurface, aerobic/anaerobic, and anaerobic on the bottom. The overallfunction is to remove residual nitrogen and phosphorus in the systemthrough the use of phototrophic microorganisms, while simultaneouslyconsuming CO₂ and creating O₂ to aid in the breakdown of residualeffluent from anaerobic digestion. These ponds can be sized individuallyfor each implementation or user application.

THE FIGURES

FIG. 1 shows schematically an embodiment of a water remediation inaccordance with one aspect of the invention.

FIGS. 2A and 2B show, respectively, a cross-sectional side view and anexploded view of an embodiment of a photobioreactor having a singledraft tube in accordance with one aspect of the invention.

FIG. 2C shows an alternative embodiment of a photobioreactor having aplurality of draft tubes.

FIG. 2D shows another embodiment of a photobioreactor utilizing parallelplates with light rods in accordance with an aspect of the invention.

FIG. 2E shows an alternative embodiment of a photobioreactor utilizingparallel plates and external illumination.

FIG. 2F shows in flow diagram form the operation of the photobioreactorin accordance with an aspect of the invention.

FIG. 3A shows an embodiment of a light rod as used in an embodiment ofthe photobioreactor of FIGS. 2A and 2B.

FIG. 3B shows a first alternative embodiment of a light rod inaccordance with the invention.

FIG. 3C shows a second alternative embodiment of a light rod inaccordance with the invention.

FIG. 4A shows an embodiment of a nutrient system in accordance with anaspect of the invention.

FIG. 4B shows in flow diagram form the operation of an embodiment of thenutrient system in accordance with one aspect of the invention.

FIGS. 5A-5B show an embodiment of a multiphasic pond in accordance withan aspect of the invention.

FIG. 6 shows an embodiment of a bubbler in accordance with an aspect ofthe invention.

FIG. 7 shows in schematic form an alternative embodiment of a wastewaterremediation system in accordance with an aspect of the invention.

FIGS. 8A-8B show in system and flow diagram forms a concentrator processin accordance with an aspect of the invention.

FIG. 9 illustrates in generalized flow diagram form an embodiment of asoft fail process in accordance with an aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, a bioremediation system in accordance withone aspect of the invention comprises a photobioreactor or PBR 10,described in greater detail hereinafter, which receives a nutrientstream from a nutrient system 15. The PBR provides an optimizedenvironment for the growth of highly concentrated algae. The algae fromthe PBR 10 is supplied via a conduit 20 to a wastewater pond or lagoon25, which, in some but not necessarily all embodiments, is a multiphasicpond as discussed in connection with FIG. 5. The pond or lagoon 25receives organic waste 30, and, in many embodiments, can also receiveatmospheric CO₂ as indicated at 35.

The wastewater pond or lagoon 25, which can cover less than an acre totens or hundreds of acres and could even be an open water area such as alake or bay given sufficiently large algae supplies, comprises in someembodiments a relatively shallow pond having at least one remediationstrata and, in the case of multiphasic ponds, a plurality of strata. Asexplained in greater detail hereinafter, the algae from the PBR areprovided to the lagoon in doses sufficient to inoculate the lagoon; thatis, to provide enough algae to the lagoon that the natural conditions inthe lagoon will permit the algae to thrive for a reasonable period oftime, propagating naturally. The algae typically, although notnecessarily, operate symbiotically with bacteria with which theycommingle in the lagoon. In the embodiment in which both algae andbacteria are present, bacterial action reduces BOD and TSS (TotalSuspended Solids) and reduces nitrogen while producing CO₂. Compared tothe bacteria, algae reduce BOD, TSS, and nitrogen to a lesser extent,and substantially reduce phosphorus, all while producing oxygen. Thissymbiotic relationship, in which the bacteria produce CO₂ consumed bythe algae as the algae produce O₂ consumed by the bacteria,significantly accelerates the activity of both organisms. (In addition,some CO₂ and O₂ come in from the atmosphere.)

By moving the wastewater through the pond or lagoon at a suitable rate,to ensure sufficient mixing, to maintain homogeneity of the waterchemistry & temperature, as well as maintain suspension of the algae andbacteria, the outflow from the pond 25 is substantially remediated.Optionally, a final treatment 40 can be provided, in the form of analgae separation step and/or a maturation or clarification stage. Analgae separation step permits collection of the algae biomass forvalue-added applications (e.g. fertilizer). A maturation pond,constructed wetland, or similar solution would promote settling of thealgae and further reduction of nitrates and phosphates. In addition, insome embodiments automated feedback, indicated at 45, can be providedwhich determines the water quality of the outflow and accordinglyadjusts the level of inoculation to ensure that proper levels of waterquality are achieved and maintained. In systems where no final treatmentstep is performed, the water quality of the output of the pond 25 isused to provide feedback.

Referring next to FIGS. 2A and 2B, an embodiment of the photobioreactorof the present invention can be better understood, shown incross-sectional and exploded perspective views, respectively. A drafttube 200 is centrally disposed within a housing or tank 205. In anembodiment, the tank has a useful capacity of about 50 gallons with adiameter of approximately 22 inches, while the draft tube has a capacityof approximately 4.5 gallons and a diameter of approximately seveninches. The relative volumes and diameters of the tank 200 and drafttube 205 can vary substantially, although in at least some embodiments adraft tube diameter of five to twenty-five percent of the tank diameterhas been found useful. In some embodiments, the tank is sized to have aheight/diameter ratio approximately 1.5:1, although this ratio is notlimiting and the relative dimensions of the tank can vary significantly.

Arranged within the draft tube are one or more light rods or pipes 210,as described in greater detail in connection with FIG. 3A. A pluralityof light pipes 210 are also arranged within the housing 205 and aroundthe outside of the tube 200, The exact number of light pipes 210 bothwithin the diffusion tube and arranged outside of the diffusion tube canvary depending upon the size of the tank and the particularimplementation of the invention. In general, it is desirable to spacethe light pipes apart by approximately twice the absorption distance ofthe light emitted from the light pipes. In an embodiment, for example,the light pipes are spaced approximately 10-15 centimeters apart,although the exact dimensions can vary depending on numerous factors,including the type of algae, the level of algae concentration desiredfor a particular PBR, and the wavelength and power of the LED'sproviding light to the light pipes.

The housing 205 contains growth media 215, as described in greaterdetail in connection with FIG. 4, together with algae selected to beappropriate for the particular remediation system. Light of one or morepreselected wavelengths, appropriate to facilitate growth of theselected algae, is supplied by one or more LED's 220 or similar lightsources associated with each of the light pipes 210. While LED's are thepreferred light source for many embodiments, other light sources areacceptable for some embodiments, including lasers, diode lasers, diodepumped solid state lasers, diode pumped fiber lasers, high intensitydischarge lamps and other lamps, infrared sources converted towavelengths appropriate for the particular strains of algae, or evensunlight coupled to the light pipes using heliostats or similar devices.For convenience, the light source will be described herein as “LED”, butis to be understood as meaning any light source appropriate for theparticular implementation of the invention. The LED's 220 are disposedat the top thereof, typically above the top of the media, where lightemitted from the LED's is transmitted down the light pipe through acoupling 225. In some embodiments, multiple LED's can be used to emitlight of different wavelengths along a single rod, a single LED can emitmultiple wavelengths along a single rod, or some rods can have LED'semitting a first wavelength while other rods have LED's emitting otherwavelengths. In addition, in some embodiments various dyes can be usedin the rods to convert light of one wavelength to another moreappropriate for the strains of algae being grown in the tank.

Each of the light pipes can also include a homogenizer or mixer, asshown in FIG. 3A, to improve spatial uniformity in the light pipes,although the homogenizer is not required for all implementations. Eachof the LED's can have associated therewith a heat sink or heat exchanger230, to keep the LED's at an appropriate operating temperature. In atleast some implementations, it is desirable to cool the LED'ssufficiently that heat from the LED's does not adversely affect thegrowth of the algae within the tank. To facilitate cooling of the LED's,one or more fans 235 can be positioned at an orifice in an air dome 240,with a ventilation gap 245 disposed between the air dome 240 and a tanklid 250 to allow air to exit. In general, the purpose of the light pipes210 is to transit light from the LED's as uniformly as possiblethroughout the tank, to encourage algae growth at all levels within thetank, while not transmitting the heat from the LED's into the tank andnot impeding fluid flow within the tank.

In an embodiment, the light rods are supported by a tank lid 250, whichhas orifices 255 therethrough. Each of the light pipes 210 slidesthrough an orifice 255 so that the majority of the light pipe fits intothe tank 205. The lid can also provide a connection point for one ormore supports 260 for the draft tube 200, so that the top of the drafttube is maintained somewhat below the surface of the liquid in the tank,and the open bottom of the draft tube is maintained above the bottom ofthe tank.

To maximize the concentration of algae within the growth medium in thetank, the algae are typically moved or stirred gently within the tank.One technique for facilitating such slow movement is to blend CO₂ orother gas (depending on what algae is being cultured and for whatpurpose) with compressed air via a computer controlled valve 265 andblender 270. In some embodiments, no compressed air is used. Dependingupon the particular implementation, the bubbled gas can be inert withrespect to the growth medium and the culture being grown, or it canpromote the growth of the culture such as by providing a nutrient, or itcan otherwise regulate conditions in the tank such as by changing pH.The combined stream is supplied to the bottom of the draft tube 200 viaa flowmeter 275 and a diffuser 277, where the diffuser operates toconvert the gas stream into gas bubbles sized to be suitable forproviding movement of the algae. The bubbles of gas mixture entrain thegrowth media and move algae in the draft tube upward as indicated by theupward flow arrows. In an embodiment, typical bubble size is on theorder of 1 mm, but can vary significantly, in a range of 0.2 mm to 3 mm,or more.

Because the top of the draft tube is below the surface of the liquid,and also suspended above the bottom of the tank 200 preferably at adistance that facilitates a vacuum effect in at least some embodiments,the algae and growth media flow over the top of the draft tube and movedownward within the portion of the tank outside the draft tube, asindicated by the downward flow arrow. In an embodiment using a singledraft tube, a gas flow rate of 0.1-0.2 cubic feet per minute providessufficient movement of the algae, although this flow rate is notintended to be limiting. This movement promotes homogenaeity of thegrowth medium within the tank, prevents settling, and also facilitatesthe algae moving along the length of the light rods, so that the algaeare relatively uniformly illuminated by the light emitted from the lightrods throughout the volume of the tank 205, thus yielding relativelyuniform growth throughout the tank, rather than merely at the surface asfound in prior art systems. Additional growth medium can be supplied asnecessary from a medium tank, as discussed in connection with FIG. 4,via tube 280A, while seed amounts of algae are supplied via tube 280B orthrough an orifice in lid 250. The junction of the walls and bottom ofthe tank can be rounded to facilitate smooth movement of the algae andto prevent algae from clogging at what would otherwise be a sharpcorner, although such rounding is not necessary in all embodiments.

To promote good algae growth, the temperature of the growth media iscontrolled by means of a thermal control jacket 285, the temperature ofwhich can be regulated by thermal control unit 290. The thermal controljacket can, for example, be formed with tubes therethough for heating orcooling fluid flow, or can be comprised of polymer heating/coolingmaterial. In addition, pH, level, and temperature, indicated by sensors295A-C, are monitored by control system 295D, typically a computer (notshown.) Once the algae concentration in the tank has reached the desiredlevel, a computer-controlled drain valve 297 permits the algae to betransferred to a lagoon or pond seen in FIG. 1 to facilitate theremediation process. In some embodiments, algae concentrations of lessthan 50 mg/L up to 5000 mg/L or more can be achieved, withconcentrations from 50 mg/L to 1000 mg/L being easily obtainable.

It will be appreciated that, while a single draft tube has been shown inFIGS. 2A-2B, multiple draft tubes can be used and may be desirable intanks having a larger diameter, as shown in FIG. 2C. In general, havinga plurality of smaller diameter draft tubes distributed around a largertank will provide better fluid flow, especially near the edge of thetank, than a single tube of equivalent flow in the middle of a tank. Inan embodiment, a gas flow of approximately 0.1 to 0.2 cubic feet perminute per draft tube provides sufficient stirring and movement of thealgae and growth medium within the tank, where the combined diameters ofthe draft tubes comprises approximately five to twenty-five percent ofthe total surface area of fluid within the tank.

In the alternative embodiment of FIG. 2C, which is a cross-sectionalview of a photobioreactor in which like numerals refer to like elementsfrom FIG. 2A, a plurality of draft tubes 200 are disposed within thetank 205. Although only two such draft tubes 200 are shown, the designillustrated can accommodate a large range of draft tubes. Thus, forexample, between one and four draft tubes are desirable for someembodiments where the tank is approximately 150 gallons, while anembodiment using a 300 gallon tank uses five draft tubes. The foregoingnumbers are exemplary only and are not limiting. In general the numberand placement of the draft tubes is intended to facilitate appropriateupward and downward flow of the algae-laden medium as described beforein connection with FIGS. 2A-2B, where the algae is permitted to growthroughout the volume of the tank, rather than just at the surface as inprior art designs.

Referring next to FIG. 2D, which shows in perspective view with atransparent front wall a still further alternative embodiment of the PBRshown in FIG. 2A, it will be appreciated that the shape of the tank neednot be round, and in fact can be any shape that permits sufficient lightto reach the volume of algae growing in the medium. For clarity,elements with like functionality are again shown with the same referencenumerals used in FIG. 2A, and, for clarity, many elements with identicalfunctionality to those discussed in connection with FIG. 2A are omitted.Thus, the perimeter of the tank 205 shown in the embodiment of FIG. 2Dis rectangular, with one or more baffles 2100 arrayed within the tankand extending from below the surface of the fluid to a distance abovethe bottom of the tank. The baffles thus create spaces having the samefunctionality as draft tubes 200. By placing diffusers 2110 withappropriate gas flow at the bottom of alternating baffled spaces, thedesired upward and downward flows are created within alternating baffledspaces in the tank, as shown by the flow arrows. As with the PBR of FIG.2A, a plurality of light pipes are disposed within the tank through alid 2115. Although a linear array of light pipes 210 is shown in FIG.2D, it will be apparent that the number and placement of the light pipeswill vary with the dimensions of the tank 200, and need not be linear.

It will also be understood, from the description of FIGS. 2A-2C, thatthe wall-to-wall baffles shown in FIG. 2D are not required in allembodiments, and instead can be replaced with draft tubes as shown inFIGS. 2A-2C. In addition, the width of the tank can be varied asdesired, with multiple draft tubes and multiple light pipes arrayed inaccordance with the teachings given in connection with FIGS. 2A-2C. Itwill also be appreciated that, in some embodiments, light pipes can beplaced in the corners of the tank, to prevent a fall-off in illuminationat the corners, although such positioning could in some instances resultin a less efficient use of the light from the corner light pipes. Addinga reflector behind the light pipe can reduce the loss. In addition, withlight pipes positioned in the corners, the fluid flow in the corners isdecreased and dead spots may occur.

Turning next to FIG. 2E, which shows a still further alternativeembodiment of a PBR in accordance with an aspect of the invention, insome embodiments the light pipes 210 can be replaced with externallypositioned LED's or equivalent light sources 2105. A thermal jacket 2110is still provided, with orifices therethrough to accommodate theplacement of the LED's 2105. Heat sinks 2115 are provided in at leastsome arrangements, and a cover [not shown] can be provided to controlthe air flow through the heat sinks, effectively creating a plenum. Aswith the design of FIG. 2D, baffles or draft tubes are disposed withinthe tank to create the appropriate flow of the algae and growth medium.The remaining elements of FIG. 2A (such as light pipes, controls, etc.)are not repeated in FIG. 2E for clarity, but would be included asappropriate in implementations of the embodiment shown in FIG. 2E. Sincethe light sources 2105 are external for the embodiment of FIG. 2E, thewidth of the tank is preferably constrained to ensure good illuminationthroughout the volume of the algae and medium flowing within the tank,and thus the width of the tank is typically at most a few inches. In atleast some embodiments, LED's 2105 are disposed on both sides of thetank.

Referring next to FIG. 2F, the process flow for growing algae within thePBR's shown in FIG. 2A-2D can be better appreciated. Growth medium issupplied to the growing tank 2200 via tube 2205 from the mediumpreparation system (FIG. 4), either manually or under computer control2207, as indicated by a level sensor 2210. Seed amounts of the selectedspecies, one or more, of algae are added via tube 2215, again eithermanually or under computer control, or through one of the orifices inthe tank lid prior to inserting the associated light pipe. Illumination2220 is enabled from the control system, and the climate control sleeve2225, or thermal jacket, brings the growth medium in the tank to atemperature appropriate for growing the algae within the tank, asmonitored by temperature sensor 2230. The control system blends gasessuch as CO2, air, nitrogen, or other gases, via solenoid valve 2235 andblender 2240, and throttles the volume of gas supplied to the tank viaflowmeter 2245. The volume of gas is controlled by the control systemboth for purposes of setting the pH, as monitored by pH sensor 2250, andfor the purpose of ensuring proper flow within the tank. Depending uponthe constituents in the growth medium, the species of algae, and thebioproducts desired to be produced by the algae, various other sensorsare monitored by the control system, for example phosphate levels 2255,nitrate levels 2260, dissolved O₂ 2265, and turbidity 2270. In additionto turbidity as a method for monitoring culture density, a colorimeterand/or a chlorophyll fluorescence probe can be used. When it isdesirable to remove the algae and associated bioproducts from the tank,either manually or via the control system a valve 2275 is opened and thealgae-laden fluid is removed from the tank via outlet 2280, either to besupplied to bioremediation lagoons or ponds, or otherwise used ordisposed of.

Referring next to FIGS. 3A-3C, various embodiments of the light pipe ofthe present invention can be appreciated in greater detail. Referringfirst to FIG. 3A, an exploded view of an embodiment of a light pipe isshown. A clear rod 300, sized of a length to permit the rod to reachsubstantially to the bottom of a tank of a photobioreactor, comprises aseries of alternating frosted and unfrosted sections 305 and 310. Therod 300 is typically comprised of acrylic or other polymer, or any othersuitable material which is optically clear at the wavelength of thelight emitted by one or more LED's 315 and capable of having a surfacetexture created on portions thereof to create the frosted and unfrostedsections 305 and 310. As noted before, the LED's can be of multiplewavelengths, with different wavelengths emitted from each rod, or allrods emitting multiple wavelengths, or all rods emitting the samewavelength. It is noted that, while the foregoing describes a singlewavelength, those skilled in the art will recognize that, in thiscontext, “wavelength” is more accurately a wavelength band, as LED'semit a spectral spread, where the center wavelength is described as the“wavelength” of the LED. Also, as noted previously, dyes can be used inor on the rods to convert light of a wavelength generated by the LED'sto light of a different wavelength suited to the algae.

The LED's 315 are mounted in a mounting block 320, which is thermallycoupled to a heatsink 325 depending on the heat generated by the LED's315. In some embodiments, it is desirable to provide spatially uniformlight from the LED's to the rods 300, in which case a homogenizer 330can be disposed in the optical path between the output of the LED's 315and the input 335 of the rod 300. The homogenizer 330 typically has anon-circular cross-section throughout most or all of its length andutilizes internal reflection, including total internal reflectiondepending upon the material used, to create spatial uniformity of thelight at the output of the homogenizer. The input face 335 of thehomogenizer 330 is typically sized so that its input dimensions aresubstantially matched to the output of the LED's, thereby allowing thehomogenizer to capture all or nearly all of the light output of theLED's. Similarly, the dimensions of the output face of the homogenizerare sized to substantially match the input of the rod 300, so that theloss of light at the transition from the homogenizer to the rod isminimized. It is not necessary that the output of the homogenizer becongruent with either the output of the LED's or the input of the lightrod. In the case of the output of the LED's, the input face of thehomogenizer can be larger. In the case of the input to the light rod,the output face of the homogenizer can, for example, be a square withits corners intersecting or contained within the circular face of therod 300, or can be any other shape reasonably contained within butsubstantially covering the input face of the rod 300, althoughhomogenizers with an odd number of sides offer improved performance insome instances.

In an important aspect of the light rods 300, the arrangement of frostedand unfrosted sections 305 and 310 control the location along its lengthand amount of light emitted from the rod. Light entering the input tothe rod is transmitted along the unfrosted sections by total internalreflection. However, at each frosted section, at least some of the lightstriking the sidewall of the rod is emitted, or coupled, from the rod.The rod, which may have any cross-section that permits total internalreflection, can have a uniform cross-section along its length, or canmonotonically decrease in size. In addition, the distal end 340 of therod 300 can either be rounded and frosted to prevent light loss, or canbe mirrored to cause the light to be retroreflected back up the rod,allowing transmission through the sidewall of the rod as describedabove. Because the end segment of the rod is a special case, where realcoupling can be significantly less than theoretical coupling due to theexponential decay of the light, such mirroring or rounding and frostingcan increase actual coupling to a reasonable approximation oftheoretical coupling.

In at least some embodiments, the length of the frosted sectionincreases relative to the length of the adjacent unfrosted section foreach successive portion of the rod. In some arrangements, thecombination of an unfrosted section and the adjacent frosted section canbe thought of as a single segment 345, and the segment length remainsthe same along the length of the rod while the relative length of thefrosted section within each segment increases for each successivesegment. The amount of light transmitted by each frosted section isproportional to its length, and so the relative lengths of the variousfrosted sections can be expressed mathematically. Where z represents thelocation along the rod of length L, and P(z) represents the intensity ofthe light in the rod as a function of z, and the strength of thecoupling due to the frosting can be continuously varied along the lengthof the rod in a controlled manner by varying the depth, shape and/orperiodicity of the grooves in the frosting, then α(z) can be a couplingcoefficient that describes the strength of the fractional coupling ofthe light per unit length from the rod by the frosting as a function ofz. In addition, let Q(z) be the light power coupled out of the rod perunit length at a particular distance z along the rod. ThusQ(z)=α(z)P(z), and the objective is to determine the function α(z) thatwill produce the desired uniform distribution of light Q(z) coupled outof the rod at the various frosted sections

For incoherent light and assuming conservation of energy, we have

dP(z)/dz=−Q(z)=−α(z)P(z)  [Eq. (1)]

with the boundary condition P(0)=P_(o).Solving for the α(z) that will produce a uniform Q(z) in Eq. (1), Q(z)is set equal to Q_(o) as is the boundary condition P(L)=0. The solutionsare:

Q(z)=Q _(o) =P _(o) /L

P(z)=P _(o)(1−z/L)

α(z)=L ⁻¹(1−z/L)⁻¹  [Eqs. (2)]

It will be appreciated that the dynamic range that can be achieved forα(z) is limited in real systems, and there will be some maximum valueα_(max) that cannot be exceeded. Thus the high values of α(z) as z/Lapproaches 1 prescribed by Eqs (2) cannot be obtained and there will besome deviation from ideal behavior. This will manifest itself as a dipin the value of Q(z), the light power coupled out per unit length nearthe very end of the rod.

In those cases where the span of values that can be achieved for α(z)needs to be adjusted higher or lower, it is possible to do such byselecting a different diameter for the light rod. This will alter thenumber of reflections each light ray will undergo per unit length of therod and thus, assuming that the properties of the frosting do notchange, α(z) will scale inversely proportionally to the rod diameter.For rods having N segments of uniform length, where F_(i) represents thefractional light power coupled out of the i^(th) segment and index i=1at the first segment and equals N at the last segment, the aboveequations simplify to

Q _(i) =Q _(o) =P _(o) /N

P _(i) =P _(o)(N+1−i)/N

F _(i)=(N+1−i)⁻¹  [Eqs. (3)]

Following are tables that present the entire solutions of Eqs. 3 forN=2, N=5, N=10, and N=20, where

i P_(i) Q_(i) P_(i+1) F_(i) For N = 2: 1 1.0000 0.5000 0.5000 50.00% 20.5000 0.5000 0.0000 100.00% For N = 5: 1 1.0000 0.2000 0.8000 20.00% 20.8000 0.2000 0.6000 25.00% 3 0.6000 0.2000 0.4000 33.33% 4 0.40000.2000 0.2000 50.00% 5 0.2000 0.2000 0.0000 100.00% For N = 10: 1 1.00000.1000 0.9000 10.00% 2 0.9000 0.1000 0.8000 11.11% 3 0.8000 0.10000.7000 12.50% 4 0.7000 0.1000 0.6000 14.29% 5 0.6000 0.1000 0.500016.67% 6 0.5000 0.1000 0.4000 20.00% 7 0.4000 0.1000 0.3000 25.00% 80.3000 0.1000 0.2000 33.33% 9 0.2000 0.1000 0.1000 50.00% 10 0.10000.1000 0.0000 100.00% For N = 20 1 1.0000 0.0500 0.9500 5.00% 2 0.95000.0500 0.9000 5.26% 3 0.9000 0.0500 0.8500 5.56% 4 0.8500 0.0500 0.80005.88% 5 0.8000 0.0500 0.7500 6.25% 6 0.7500 0.0500 0.7000 6.67% 7 0.70000.0500 0.6500 7.14% 8 0.6500 0.0500 0.6000 7.69% 9 0.6000 0.0500 0.55008.33% 10 0.5500 0.0500 0.5000 9.09% 11 0.5000 0.0500 0.4500 10.00% 120.4500 0.0500 0.4000 11.11% 13 0.4000 0.0500 0.3500 12.50% 14 0.35000.0500 0.3000 14.29% 15 0.3000 0.0500 0.2500 16.67% 16 0.2500 0.05000.2000 20.00% 17 0.2000 0.0500 0.1500 25.00% 18 0.1500 0.0500 0.100033.33% 19 0.1000 0.0500 0.0500 50.00% 20 0.0500 0.0500 0.0000 100.00% i= segment index; P_(i) = incident light power; Q_(i) = coupled out lightpower; P_(i+1) = transmitted light power; and F_(i) = fractional lightpower coupled out

As noted previously, the distal end (the last segment) is a specialcase, where beveling, rounding or other shaping can be used to achievenearly 100% coupling as well as coupling out any light propagatingballistically down the rod 300.

For segments of uneven length, the outcome is substantially the same,where the light output of any segment is determined by comparing thelength of a given segment to the average segment length. Statedmathematically, let L_(i) be the physical length of the ith segment.Since the overall length of the rod is L,

$\begin{matrix}{L = {\sum\limits_{i = 1}^{N}L_{i}}} & \left\lbrack {{Eq}.\mspace{14mu} (4)} \right\rbrack\end{matrix}$

where Σ denotes the sum over all segments, that is all values of i from1 thru N. In order to produce a distribution of coupled out power thatis uniform over the physical length of the rod, it is necessary to scalethe values of Q_(i), the light power that is coupled out of the i^(th)segment, by L_(i)/L_(av) where L_(av) is the average segment lengthgiven by L_(av)=L/N. The solutions of Eq. (3) then become:

$\begin{matrix}{{Q_{i} = {{\left( {L_{i}/L_{av}} \right)\left( {P_{o}/N} \right)} = {\left( {L_{i}/L} \right)P_{o}}}}{P_{i} = {P_{o} - {\sum\limits_{k = 1}^{i - 1}Q_{k}}}}{{F_{i} = {Q_{i}/P_{i}}},}} & \left\lbrack {{Eqs}.\mspace{14mu} (5)} \right\rbrack\end{matrix}$

where Σ denotes the sum of all of the values of Q_(k) coupled outpreviously in segments k=1 thru k=i−1. The numerical evaluations of Eqs.(5) are easily obtained using a spreadsheet that starts with the knownvalues for i=1 of Q₁=(L₁/L)P_(o), P₁=P_(o), and F₁=L₁/L and then fillsin each line for higher values of i based on the values from theproceeding line.

As a specific example, consider the case of a rod with 10 segments, nineof which have length 1 and one of which (the 3rd segment) has length 3.The results are shown in the table below. Examination of the resultsshows that the coupled out light power Q₃ for the 3rd segment is threetimes as high as the power coupled out by the other segments, asexpected.

N = 10 F_(i) i L_(i) P_(i) Q_(i) P_(i+1) (Fractional (Segment (Segment(Incident (Coupled Out (Transmitted Light Power Index) Length)L_(i)/L_(av) Light Power) Light Power) Light Power) Coupled Out) 1 10.08333 1.0000 0.0833 0.9167 8.33% 2 1 0.08333 0.9167 0.0833 0.83339.09% 3 3 0.25000 0.8333 0.2500 0.5833 30.00% 4 1 0.08333 0.5833 0.08330.5000 14.29% 5 1 0.08333 0.5000 0.0833 0.4167 16.67% 6 1 0.08333 0.41670.0833 0.3333 20.00% 7 1 0.08333 0.3333 0.0833 0.2500 25.00% 8 1 0.083330.2500 0.0833 0.1667 33.33% 9 1 0.08333 0.1667 0.0833 0.0833 50.00% 10 10.08333 0.0833 0.0833 0.0000 100.00%

The practical effects of the mathematical descriptions given above canbe appreciated from FIGS. 3B and 3C, which show, respectively, whereFIG. 3B shows segments of uniform length while FIG. 3C shows segments ofuneven length, in which segment N₃ is twice as long as the othersegments. The hatched portions represent the frosted sections of eachsegment, where light is coupled out. Thus, for FIG. 3B and equal lengthsegments, the coupling is as shown in the table for N=5. But, for thedesign of FIG. 3C, where N₃ is twice as long as the other segments, theequivalent number of segments is six, and N₃ couples out ⅓+¼ of thelight, or a total of 48.3% across the longer segment.

It will be appreciated by those skilled in the art that, while the rod300 is shown as a consistent diameter down its length, other shapes andcross-sections of light rods are also acceptable. Thus, for example,tapered light rods can also be used in at least some embodiments.Likewise, the light rod 300 need not be straight in some embodiments,and instead can be curved in any suitable arrangement. Non-circularcross-sections, while harder to manufacture in some cases, may offermore uniform light distribution characteristics along the length of therod in some embodiments. Further, while the frosting is assumed to beidentical for each segment in the foregoing calculations and examples,in some embodiments it is desirable to vary the optical properties ofthe frosting at each segment. Such variations in the frosting provides ameans to extend the dynamic range over which the coupling can be varied.Likewise, the variation in the frosting does not need to be continuous.Having a few discrete values, such as “weak”, “medium” and “strong”,offers benefit in some embodiments, while continuously variable frostingallows fine tuning of the fractional power coupled out by each segment.

Referring next to FIGS. 4A-4B, the nutrient system for supplying aregulated nutrient stream to the algae growing in the PBR can be betterappreciated. A plurality of carboys 400A-n, each containing a componentof a predetermined nutrient mix appropriate for a specific strain ofalgae, are associated with a plurality of metering pumps 410A-n, each ofwhich is computer controlled. The metering pumps thus supply a desiredmix of nutrients into a mixing tank 420, which receives water 425 via acomputer controlled valve 430. A number of filters 435 and 440 can alsobe installed between the inlet water and the mixing tank; for example,five micron and carbon filters, respectively.

The outlet of the mixing tank is supplied to a computer controlled pump445, which supplies the mixed nutrient stream to a computer controlledvalve 450. The valve 450 directs the nutrient mix either to berecirculated in the tank via recirculation line 460 or to be supplied toan associated PBR or group of PBR's as indicated at 470. Filters 455 and465, which can, for example, be two micron filters, can be provided onthe recirculate and PBR tank lines, respectively.

With particular reference to FIG. 4B, the process for preparing thegrowth medium using the system of FIG. 4A can be better appreciated. Theprocess starts at step 4000, and at step 4005 the tank 420 is filledwith water to a predetermined level as determined by level sensor 475,after which the pump 445 is turned on and configured to recirculate thetank contents by valve 450 as shown at step 4010. At step 4015, thewater in the tank is heated by heater 485 to a predetermined temperatureas measured by temperature sensor 480. The nutrient constituentsappropriate for the particular growth medium being developed are thesupplied to the tank from carboys 400A-n via their associated meteringpumps 410A-n at step 4020. The constituents of the growth medium canvary with the particular algae strain for which the growth medium isintended. The mix of water and nutrients is then circulated, as shown atstep 4025, until the nutrients are uniformly distributed, after whichthe growth medium is supplied to an associated PBR at step 4030. Theprocess either completes, as shown at step 4030, or loops back to step4005 to begin again.

Referring next to FIGS. 5A-5B, a multi-phasic pond in accordance with anaspect of the invention can be better appreciated. FIG. 5A shows a pond500 with a paddlewheel 505 in top plan view, and also shows the locationacross which the cross-sectional view of FIG. 5B is taken. FIG. 5B showsthe various strata of the pond. In particular, an anaerobic zone 510 islocated at the bottom of the pond. An anaerobic/aerobic transition zone515 is located above the anaerobic zone, at the top of which aredisposed one or more CO₂ supply tubes 520. The CO₂ supply tubes aretypically porous tubing for distributing CO₂ substantially uniformlyacross at least a substantial portion of the pond 500. The CO2 isutilized by aerobic bacteria in an aerobic zone 525. The paddlewheel 505creates a flow, shown from left to right in exemplary FIG. 5B, such thatmixing of the effluent being remediated across the various zones isfacilitated. In ponds or lagoons having only an aerobic zone, the tubes520 can be arrayed on the bottom of the pond or lagoon.

Referring next to FIG. 6, a bubble column with internal lighting inaccordance with an aspect of the invention can be better appreciated. Insome instances, it is desirable to study the growth of algae or otherflora in a carefully controlled environment, such as for research. Insuch instances, it is sometimes desirable to ensure appropriatelycontrolled illumination and nutrient supplies, while not circulating thegrowth medium or growing species in a larger tank. For suchimplementations, the bubble column of FIG. 6 is appropriate. A tank 600has a sealed bottom and has disposed therein at least one light rod 605of the type described in connection with FIGS. 3A-3C. Although a roundtank 600 is shown, the tank need not be round in all instances, andinstead can be any convenient shape. The light rod can be centrallydisposed or disposed asymmetrically, and can be configured together withthe shape of the tank to provide whatever uniformity of illumination orlack thereof is desired. In an embodiment, the position of the light rodcan be varied within the tank to facilitate different illuminationpatterns. Compressed air or other gases, utilized in the mannerdescribed in connection with FIGS. 2A-2B et seq., are supplied to adiffuser 610 located at the bottom of the tank via a tube 615, which caneither enter the tank from the bottom or down an inside wall as shown.The diffuser can be configured to supply gas uniformly across the bottomof the tank or in any desired pattern, but the sole agitation and mixingis through the upward movement of the bubbles through the algae-ladenmedium, since there is no larger tank for creating the upward anddownward flows of the systems shown in FIGS. 2A-2F. In at least someembodiments, a supply tube 620 is provided by which algae can beintroduced to the column. The supply tube 620 can be located at anyconvenient position on the tank, including a lid 625, or an orifice inthe lid through which the light rod 605 passes, a sidewall, or thebottom of the tank.

In operation, the bubble column is filled with a growth medium, andalgae strains are introduced. A gas mixture appropriate for theparticular study being conducted is introduced via the diffuser, and theresulting bubbles entrain the algae as described above. However, becausethe bubble column is not contained within an outer housing or tank, thefluid levels are typically maintained at levels below overflowing inmost embodiments.

Referring next to FIG. 7, an alternative embodiment of a wastewaterremediation system in accordance with an aspect of the invention isshown in schematic form. Organic waste 700 is supplied to an anaerobicdigester 705, which begins the breakdown process and generates methane710 and an effluent stream 715, comprised in part of CO₂, nitrogen,phosphorus and other constituents. The effluent is supplied to amultiphasic pond 720, together with water 725 as needed. The methaneprovides fuel for a generator/boiler 730, which generates heat 735 thatis supplied back to the anaerobic digester 705. The generator 730 alsoprovides CO₂ 740 to a photobioreactor 745, typically constructed inaccordance with the aforementioned teachings, as well as the multiphasicpond 720. The generator 730 also supplies electricity and heat 750 toboth the PBR 745 and the multiphasic pond 720, and may in someimplementations supply additional electricity at 775. The pond 720receives additional atmospheric CO2 at 760, if needed, and outputsremediated wastewater. The remediated wastewater can then be given anoptional final treatment, as shown at 765, such as an ultravioletpolish, carbon filtration, or other remediation step. In someembodiments, the pond 720 can also generate usable biomass as shown at770.

Referring next to FIGS. 8A-8B, a concentrator system and process can bebetter appreciated. Some species of algae grow best under one set ofconditions, but produce desired products more rapidly under differentconditions. One example is algae that grows best when supplied withnitrogenous nutrients, but produces higher concentrations of lipids whendeprived of nitrogenous nutrients. To maximize the productivity of asystem, it is desirable to create different growing conditions toachieve the different goals, and also to achieve the transition asquickly as possible. As shown in FIGS. 8A-8B, algae of a desired speciesis grown to a desired density at step 800 in a PBR 850 using a firstgrowth medium designed to achieve high algae density as promptly aspossible. Once the density level has been achieved, at least a portionof the algae is transferred to a concentrator tank 855, as shown at step810, typically although not necessarily through a computer-controlledvalve 860. The transfer process requires that a substantial amount ofthe first growth medium be transferred with the algae, to prevent damageto the algae.

After transfer to the concentrator tank 855, the combination of algaeand the first growth medium are allowed to settle as shown at step 815,causing the growth medium, which is largely water, to clarify. Then, atstep 820, the clarified growth medium is removed, either from the top ofthe concentrator or any other suitable location that will not removeand/or damage the algae within the concentrator 855. It will beappreciated that not all of the first growth medium can be removed, buta significant percentage, in the range of 75%, can be removed withoutdamaging the algae. Then, at step 825, the remaining growth medium andthe algae are transferred to a blooming tank 870 through a valve 875,also typically but not necessarily computer-controlled. A second growthmedium is used in the blooming tank 870, formulated to stimulatedevelopment of the desired products, as shown at step 830. It will beappreciated that, in at least some embodiments, the second growth mediumis added to the blooming tank in advance of the transfer of the algaeinto the blooming tank, to minimize physical damage to the algae duringtransfer, although these steps can be reversed depending upon theparticular algae, the amount of first growth medium remaining after step820, and the trauma likely to be suffered by the algae during thetransfer process. To facilitate a smooth transfer with minimal trauma tothe algae, a concentrator tank 855 having a funnel-shaped lower portioncan be used, where the algae settles in the funnel-shaped portion bothto permit easy removal of the first growth medium and to permit easytransfer to the blooming tank. Once the algae and second growth mediumare combined in the blooming tank 870, the process waits until adequateamounts of the desired products are produced by the algae, at whichpoint those products are removed for further use, and shown at step 835.The blooming tank can be configured in substantially the same way as thePBR 850, and in some implementations the PBR 850 can be re-used as theblooming tank.

In use, the first growth medium can, for example, be nitrogen-rich andthus encourage rapid growth of selected algae. The transfer to theconcentrator and removal of the first growth medium rapidly reduces thelevels of nitrogen and other nutrients, including trace elements, in thealgae. Then, the second growth medium can be, for example, pure water,or nitrogen-depleted. At this point the selected algae begin to producelipids or other products which can be used, for example, as biofuels.The result, and benefit, of the concentration process is that it rapidlyaccelerates the depletion of nutrients in the growth medium which, inturn, accelerates the generation of the desired products. For example,if the concentration process were not used, the desired depletion ofnitrogen in the first growth medium could take in the range of ten days,during which time the rate of algae growth would be substantiallysub-optimal, while at the same time the algae would not be producing thedesired levels of usable products. By comparison, the concentrationprocess of the present invention can be accomplished within minutes or,at most, hours, such that the beginning of production occurs much morerapidly, resulting in increased efficiency and lower operating costs.

Inevitably, power failures will occur regardless of the quality ofbackup systems. Algae is somewhat fragile, and loss of power forextended periods will kill the algae being grown in the PBR's andblooming tanks. To prevent unnecessary loss of algae in the event of apower failure, it is desirable to provide a soft-fail sequence by whichthe life of the algae is prolonged for as long as possible. A soft-failprocess is described in FIG. 9, where a check is made at step 905 todetermine whether power is at proper levels. If yes, the process loopsso that checking for power failures is essentially continuous. If poweris not at proper levels, the process advances to step 910 and anypreparation of growth medium is halted, as is any algae transfer ordischarge. In addition, as shown at step 915, the LED's are turned off.Further, at step 920, the temperature tolerances set into the controlsystem are automatically expanded. Still further, the pH target in thetank is adjusted for the particular species being grown to maximizeculture viability, as shown at step 925. Finally, the gas stream isswitched from continuous operation to intermittent, so that mixing inthe tank continues although not at the same levels. It will beappreciated that steps 910 to 930 can be performed either essentiallyconcurrently, or in stages where the time increment between each stepcan be adjusted depending upon the particular operating conditions, thestrain(s) of algae, and the projected time before power is restored.Finally, as shown at step 935, the state of the power is tested again.If power has been restored, the operations of steps 910-930 are restoredto normal conditions as shown at step 940. If power has not beenrestored, the check continues until power is restored or reserve poweris lost.

From the foregoing, it can be appreciated that new and novelbioremediation systems and methods have been described, with novelaspects regarding illumination, nutrient supply and mixing, algae growthprocesses, generation of biomass and other products, and soft failureprocesses. Having fully described a preferred embodiment of theinvention and numerous alternatives of the various aspects of theinvention, those skilled in the art will recognize, given the teachingsherein, that numerous alternatives and equivalents exist which do notdepart from the invention. It is therefore intended that the inventionnot be limited by the foregoing description, but only by the appendedclaims.

1. A bubble column for facilitating algae growth comprising a tankhaving a top surface and a bottom and adapted to contain a volume ofgrowth medium below the top surface, the growth medium having algaegrowing therein, a light pipe positioned substantially centrally withinthe tank for illuminating substantially the entire volume of growthmedium, a diffuser, adapted to receive a gas stream and positioned nearthe bottom of the tank, for creating a stream of bubbles for agitatingthe growth medium without creating fluid flow within the tank.